The conventional steering system is disclosed in DE 3714833A.
A steering wheel shaft is connected via a rotary slide valve with an input member such as a steering worm gear or a bevel gear of a servo assisted steering gear. A working piston of a hydraulic servomotor or boost device is arranged within the steering gear housing. In case of a relative rotation of the steering wheel shaft with respect to the input member, the rotary slide valve operates to apply pressure generated by a hydraulic servopump to assist the steering effort. At a power takeoff shaft an electric motor is connected with the input member of the steering gear. The electric motor is controlled by a signal from a sensor which is generated in response to rotation of the steering wheel. In the conventional servo assisted power steering system, a signal from a sensor such as a torque sensor, provides the electric motor power assist in case of hydraulic power boost failure such as low oil pressure. A torque rod arranged between the steering wheel shaft and the input memeber, supplies a relatively minor but vehicle operator noticeable reaction torque to the steering wheel. By means of the torque rod, the rotary slide valve is also turned toward its neutral position. Precise mid-point centering of the rotary slide valve and influencing the manual force or torque required by the operator by the presence of parameters such as vehicle load or speed in not possible in the conventional power assisted steering system.